Conventional power factor correction capacitors of the rolled-foil variety consist of alternate dielectric film and conducting metallic foil sheets which are wound to form a compact flattened roll or pack. Conventional means for connecting the conductive sheets of a completed capacitor unit to an external electrical circuit most often include elongated terminal strips or electrodes which are inserted between overlapping foil and dielectric film sheets. The terminal strips are typically inserted during winding of the capacitor packs. These strips project in the axial direction from the wound roll and serve to axially connect conducting foils of opposite plurality therein with external terminals on the capacitor enclosing case or tank. An example of this construction is shown in U.S. Pat. No. 3,724,043 to Eustance.
This construction has not been without problems. Premature failure of a substantial number of completed capacitor assemblies has been attributed to failure of dielectric sheets at points immediately adjacent the terminal strips. Because the terminal strips tend to slip, shift, or otherwise become dislodged and misaligned within the roll in which they are inserted, almost every terminal strip requires some movement for proper adjustment and alignment. This can result in material damage. Torn and abraded portions of the dielectric sheets have been found to fail under the electrical stress of testing and operating voltages applied to completed capacitor assemblies.
There are other short comings. The aforementioned capacitor construction requires that the capacitor pack winding machines be stopped periodically so that the terminal strips, as many as up to four pair, may be inserted within the interior of the pack being wound. The acceleration of a partially wound roll immediately after a terminal is inserted in a roll, at times, creates tension forces sufficient to scratch, scuff or otherwise damage the relatively thin overlying dielectric sheets. Thus, a significant cost savings in both labor and material damage could be realized if this intermittent winding operation could be replaced by a continuous operation.
One important disadvantage of the use of separate terminal strips is that the resistive losses are relatively high. This is because a relatively long length of foil is disposed between each strip extension. Similarly, the foil strips only make physcial contact with the foil at discrete points along the length of foil.
There are other means for connecting the two foil sections of each capacitor pack. Some capacitor designs use extended foil units. These units are characterized as having the two electrode foils in each capacitor pack axially off-set from each other and from the solid dielectric material. In the assembled pack, one electrode of each foil section extends from the upper end and the other electrode extends from the lower end. Metal, such as a solder mat, is then applied to each foil end so as to form a connection point. Where direct connection of ends of adjacent sections is desired, a single continuous solder mat is applied over the adjacent foil sections to be connected. In those instances where adjacent section ends are not to be connected, a gap is left in the applied solder mat. Isolation of adjacent section ends may be facilitated by having an insulated separator between each two sections that are to be independently connected. Such a separator extends outward from the section end sufficiently to avoid contact between foil sections. One example of this construction is provided in U.S. Pat. No. 4,442,473 to Holtzman.
Unfortunately, the solder means of connection also has problems. A disadvantage of solder connections is that the hot solder may melt the insulating film between the foils and cause contamination of the dielectric fluid. Solder connections do not always make good electrical contact. This is due to the fact that it is difficult to solder to aluminum foil. Thus, for the most part the process of attaining a good solder joint in foil capacitors is difficult and costly. Solder connections are shown in U.S. Pat. No. 3,267,343 to Rayburn. The electrical connection between capacitor packs is exemplified in FIG. 2 of U.S. Pat. No. 4,442,473 to Holtzman. As a result of this difficulty, non-conventional soldering techniques have been used. For example, electron beam welding has been suggested. This is described in U.S. Pat. No. 4,301,354 to Williams; as can be expected this process is also very expensive. U.S. Pat. No. 4,467,397 to Theil et al uses metal deposited along the edges of the terminal strips. This clearly complicates fabrication.
Thus, those skilled in the art have yet to solve the problem of manufacturing a power factor correction capacitor which is reliable in operation and is inexpensive to produce without depending upon highly skilled factory workers and without major capital investment. Clearly, the industry would welcome a design that would satisfy this long felt need.